Question

The power of a lens ‘X’ is −2.5 D. Name the lens and determine its focal length in cm. For which eye defect of vision will an optician prescribe this type of lens as a corrective lens?

Answer 1

Given: P = −2.5 D (diopters)

Formula:

Determine the focal length,

The power P of a lens is related to its focal length f (in meters) by the equation:

∴ `P = 1/f`

∴ `f = 1/2.5`

= −0.4 m

= −40 cm (or 40 cm in magnitude)

The negative sign of the power indicates that the lens is a diverging (concave) lens.

A concave lens is typically prescribed to correct myopia (nearsightedness). In myopia, the eye focuses images in front of the retina. A concave lens diverges the incoming light rays, moving the focal point back onto the retina for clear vision.